1. Field of the Invention
The present invention relates to a switching power supply controller and a switching power supply.
2. Related Background Art
A switching power supply has such properties as small size, light weight, and high efficiency and is commonly used as a power supply for microcomputers as incorporated in various devices, personal computers, and so on. These personal computers and others are progressively advancing toward lower voltage and higher processing speed, while increasing the consumption current more and more. In the switching power supply, therefore, the load current suddenly increases or decreases according to the processing load in the personal computers and others. The switching power supply possesses the property of capability of readily adapting to a wide input voltage range and is also utilized as a power supply applicable in several countries in the world or as a power supply with a wide specification setting of input voltage. The switching power supply needs to keep its output voltage constant so as to provide a target voltage according to the load of the personal computer or the like, in order to operate the personal computer or the like stably, and thus needs to insure the stable output voltage against change of the load current and input voltage. Furthermore, even if the output voltage is in a transient response state to a sudden change of the load current or the input voltage, the switching power supply must recover quickly into a stable state.
For this reason, the switching power supply is equipped with a controller such as a controller IC [Integrated Circuit] or the like of a digital control system, and this controller quickly turns switching elements such as FETs [Field Effect Transistors] or the like on and off. In order to keep the output voltage constant, the controller adopts the feedback control based on voltage mode control or current mode control to generate a PWM [Pulse Width Modulation] signal for turning the switching elements on and off on the basis of the output voltage or the like of the switching power supply.
For example, where the control is based on the current mode control by P [Proportional] control, the controller compares a current signal resulting from detection of an electric current flowing through an inductor of a smoothing circuit, with a target current signal and generates the PWM signal to provide a high signal during a period before the inductor current signal reaches the target current signal, and then provide a low signal during a period thereafter. In the current mode control, as described, the magnitude of the electric current flowing through the inductor is controlled based on the detected value of the current flowing through the inductor and the target value, thereby implementing the control to keep the output voltage constant. The target current signal is a signal obtained by subtracting the output voltage detected in the switching power supply, from the target voltage and multiplying the difference by a gain of P control.
The current mode control as described above can be implemented in either of digital control and analog control. In the digital control system, the output voltage and inductor current need to be subjected to A/D conversion from an analog signal to a digital signal, and then their digital signals are fed into the controller. Therefore, where the current mode control is implemented by the digital control, the inductor current increases or decreases according to fast on/off of the switching elements, and thus the A/D conversion causes values of digital signal after the conversion to lag behind values of actual current, thus producing error from the actual current. For this reason, the controller performs the comparison process, using the inductor current containing the time lag due to the A/D conversion, so that it fails to produce the PWM signal corresponding to the electric current actually flowing through the inductor. In this manner, the error due to the lag can be a factor to adversely affect the switching power supply required to demonstrate stable fast response. In this connection there is a controller configured to estimate the inductor current from the PWM signal generated inside the controller, instead of the digital signal (real current) after the A/D conversion, and perform the current mode control through the use of the estimated current (JP-A-2002-530036). In the technology described in this patent document, the estimated current is generated by estimating the electric current flowing through the inductor in the voltage regulator, and the output voltage of the voltage regulator is controlled constant on the basis of the estimated current.